The invention relates to electronics manufacturing equipment and, in particular, to an optical sensor for detecting objects in a manufacturing stage during a transfer operation.
In semiconductor manufacturing, integrated circuits are formed on silicon wafers during successive manufacturing stages involving formation and deposition of thin films on the wafers, together with selective removal of unwanted portions. To accomplish these operations, hundreds of individual steps are needed involving various pieces of process machinery, tools, measurement stations, ovens, cleaners and other equipment. Frequently, wafers are moved from one environment, such as an atmospheric pressure environment, to a different environment, such as a vacuum environment. To accomplish this, wafers are placed in cassettes or other carriers for bulk transfer and the carriers are passed through a bulkhead separating the two environments. Sometimes the carriers are sealed units, such as pods known as SMIFs (Standard Mechanical Interface) or FOUPs (front opening unified pod). Other times, the cassettes are transferred outside of sealed units because the material handling equipment does not need or use pods. For example, where a cassette is to be picked up by a robot arm, a cassette is merely transferred through a port in the bulkhead separating two environments.
In wafer handling using cassettes, it is desirable to have a wafer map indicating the number of wafers and position of the wafers within the slots of a cassette. In this patent application, the term xe2x80x9cwafer mapxe2x80x9d refers only to number and position of wafers in a cassette. Having such a wafer map, another piece of equipment, such as a robot arm could be commanded to go to specific locations of the cassette to pick up or deposit a wafer. Without a wafer map, the robot would have to rely on its own sensors to determine whether a wafer was loaded in a particular slot of a cassette. Wafer mapping units are known. For example, U.S. patent application Ser. No. 09/038,809, by J. Gordon et al., assigned to the assignee of the present invention, discloses a wafer mapper unit for use with a FOUP (front opening unified pod). In that application, a robot arm, or end effector, associated with a load port interface, carries a wafer mapper so that wafer information can be used in connection with wafer processing and manufacturing equipment.
Although use of a wafer mapper with a FOUP is a substantial benefit, a more fundamental need exists, namely to provide a wafer mapper for use with cassettes independent of a FOUP. Yet, because of the large number of measuring instruments already in a wafer manufacturing line, most manufacturers are not enthusiastic about adding another measuring station to an assembly line, especially a tool which might be needed for repeated measurements as wafers advance along a manufacturing line from one stage to another. Accordingly, an object of the invention was to devise a wafer mapper tool for wafer manufacturing operations which would not require much additional space in a manufacturing line, yet could be used repeatedly at various manufacturing stages.
The above object has been achieved with a wafer mapper which is associated with a load port interface in a bulkhead separating different environments of a manufacturing operation. In particular, a door in the bulkhead is fashioned with front and back panels which are linked with a wafer mapper. In one embodiment, a panel acts as a box-like housing for the mapper. At the same time, at least one of the panels must seal a port in a bulkhead of a load port interface. To accomplish this the door is hinged in a manner allowing the door to be horizontal, on the operator side of the bulkhead for loading and locking of a cassette onto the door, with the wafers within the cassette in a vertically upright position. A port cover plate is connected to the door panels by an angle bracket and a pivot. The cover plate has dimensions for sealing the port in the bulkhead. The cover plate makes an L-shape relative to the panel members so that each of the cover plate and the door panels is able to seal the port in the load port interface. Upon closing the door, the door moves to a vertical position by a 90 degree rotation, sealing the port, moving the wafers to a horizontal position on the process side of the bulkhead. The wafers are held in place since the cassette is locked in place onto the door. The cassette is open at the top and bottom, being wider at the top than at the bottom, allowing optical inspection of the cassette through the top or bottom. Since the top of the cassette is now oriented horizontally, facing the back panel of the door, the optical inspection apparatus of the wafer mapper can determine wafer positions in the cassette through a window in the back panel of the door.
The wafer mapper consists of a miniature trolley, riding on a rail and driven by a leadscrew, all mounted between the front and rear panels of the door. The trolley carries a pair of beam sources, such as semiconductor lasers, as well as mirrors and optical detectors. The beam sources and mirrors direct a pair of beams onto the mirrors, then through a window in the trolley housing onto the expected position of a wafer. If light is reflected by a wafer edge, the optical detectors will detect the reflection, recording the reflected light signal from the wafers. The trolley is driven by a motor for advancing the trolley incrementally from one end of the door, say the upper end, to the opposite end of the door and then back again. An air jet on the opposite side of the window is dragged by a magnet carried by the trolley in order to clear the window of moisture droplets in the case of wet process applications or particulates in the case of dry applications. The motor is preferably a stepper motor whose motion is precisely known relative to a starting point. In this manner the known position of the trolley may be related to light reflected from individual wafers, thereby mapping the position of wafers in the cassette. The known wafer positions are recorded by a computer and passed along to other instrumentation, such as process equipment.